The invention concerns a device for transmitting signals via induction between a transponder circuit and an interrogation circuit. For transmission of the inductive signals, the transponder circuit comprises a first coil, and the interrogation circuit comprises a second coil. The transponder circuit is placed on an object capable of rotating about at least one axis of rotation passing through the object, whereas the interrogation circuit is placed on a structure, which can be stationary, to which the object is connected.
In an automobile application, the rotating object can be for example a vehicle wheel, whereas the structure is for example the vehicle body or chassis. In such case, one could envisage the transponder circuit including at least one sensor for measuring a physical parameter. It may be a pressure sensor for measuring the vehicle tyre pressure, a temperature sensor, a force sensor, an accelerometer or any other type of sensor. The measurements made by the transponder circuit sensor or sensors can be transmitted to the interrogation circuit in the inductive signals for example by amplitude modulation.
Since the transponder circuit is placed on the rotating object, the distance separating it from the interrogation circuit is not constant during rotation of the object. Consequently, a stray amplitude modulation occurs during transmission of the inductive signal between the interrogation circuit and the transponder circuit, which can be a significant drawback. Thus, during demodulation operations in the receiver unit, it can happen that the data received in the inductive signals does not entirely correspond to the transmitted data. This interference to the transmitted data can also be dependent upon the rotational speed of the object. The higher the rotational speed, the more this interference can be significant.
By way of illustration, FIG. 1 shows in a simplified manner the influence that the rotation of an object can have on the amplitude of the inductive signals sensed by the receiver unit. Inductive signals are first of all transmitted by the transmitter unit at a determined carrier frequency and determined amplitude. The modulated data in the inductive signals are thus not shown in this Figure for the sake of simplification. Since the distance separating the receiver unit from the transmitter unit varies when the object rotates, the amplitude of the sensed inductive signals, i.e. the amplitude of the induced voltage in the receiver unit coil, changes.
In FIG. 1, this amplitude variation is illustrated by the envelope of inductive signals sensed by the receiver unit. This envelope is represented simply in sinusoidal form corresponding to a constant rotational speed of the object. However, it is clear that the shape of this envelope is not actually sinusoidal, since the amplitude of the magnetic field created by the transmitter coil does not decrease linearly with the distance separating the transmitter coil from the receiver coil.
In certain signal transmission devices, it has also been proposed to transmit high frequency signals between the transponder circuit and the interrogation circuit. The high frequency signals used avoid excessive dependence upon the rotation of the object on which the transponder circuit is mounted. However, by using a device of this type which produces high frequency signals (for example 2.45 GHz) in an automobile application, it has been demonstrated that the water can have a negative influence on the performance of the device. Moreover, since the transponder circuit transmits high frequency signals, it must be provided with its own source of energy, such as a battery. This may also be a drawback, since in this case the transponder circuit consumes energy even if the interrogation circuit is not interrogating it.